Users are only able to hang up on a call with Siri on iPhone models 11 or later. If you have an older iPhone, you will need to wait for the other person to hang up first so the call will end. On iPhone models 11 or later, there is an option in the Siri Settings that allows you to enable/disable "Call Hang-up". Note that the other person will hear you say “Hey Siri, hang up” before the call is disconnected.